1. Field of the Invention
This invention relates to improvements in a cylinder direct injection spark-ignition internal combustion engine, and more particularly to a fuel injection control technique during a time at which combustion condition in a cylinder is changed over from homogeneous charge combustion to stratified charge combustion and vice versa.
2. Description of the Prior Art
In recent years, attention has been paid to cylinder direct injection spark-ignition internal combustion engines in which fuel is directly injected from a fuel injector valve into each cylinder of the engine. These engines are configured in general as follows: Combustion condition or manner in the cylinder is controlled to be changed over from homogeneous charge combustion to stratified charge combustion and vice versa. In the homogeneous charge combustion, fuel is injected on an intake stroke to spread fuel within a combustion chamber thereby to form a homogeneous air-fuel mixture (charge) in the cylinder. In the stratified charge combustion, fuel is injected on a compression stroke to concentrically form a layer of rich air-fuel mixture around a spark plug thereby to generate a stratified air-fuel mixture (charge) in the cylinder. An example of such engines is disclosed in Japanese Patent Provisional Publication No. 59-37236.
In connection with the above engines, it has been proposed to set a combustion region for accomplishing so-called weak stratified charge combustion between a combustion region for accomplishing stratified charge combustion and a combustion region for accomplishing homogeneous charge combustion, in a map having combustion regions which are set in accordance with engine speed and engine load (torque). In this weak stratified charge combustion region, two separate fuel injections are made respectively on intake and compression strokes in each cycle of the cylinder. This weak stratified charge combustion region is set for the following reasons: If the stratified charge combustion region and the homogeneous charge combustion region are set adjacent to each other upon equivalence ratio of the adjacent regions being continuous in order to prevent stepwise torque change during a time period in which changeover of stratified charge combustion and homogeneous charge combustion is carried out, the equivalence ratio becomes unsuitable for either one of stratified charge combustion and homogeneous charge combustion around the border between the stratified charge combustion region and the homogeneous charge combustion region. As a result, at a part (near the border) of the stratified charge combustion region, air-fuel mixture around a spark plug becomes excessively rich, thereby causing misfire while increasing smoke. At a part (near the border) of the homogeneous charge combustion region, the homogeneous air-fuel mixture becomes excessively lean, thereby causing misfire while making combustion unstable.
As discussed above, in the weak stratified charge combustion region between the stratified charge combustion region and the homogeneous charge combustion region, the two separate fuel injections are made respectively on intake and compression strokes while basically accomplishing stratified charge combustion upon suitably setting spark timing of the spark plug. This suppresses formation of an excessively rich air-fuel mixture around the spark plug and suppresses increase of smoke while suppressing misfire and occurrence of unstable combustion under homogeneous charge combustion.
However, drawbacks have been encountered in the above conventional engines because no allocation rates of fuel injection quantities respectively on intake and compression strokes have not been taken into consideration. For example, there will be the possibility that the fuel injection quantity (for stratified charge combustion) on the compression stroke becomes excessively large when the equivalence ratio is relatively large (rich) thereby providing the tendency of misfire occurring, and that the fuel injection quantity (for stratified charge combustion) becomes excessively small when the equivalence ratio is relatively small (lean) thereby providing the tendency of misfire occurring.
Additionally, in the above discussed engines, the two separate fuel injections are made by dividing a predetermined quantity of fuel into two portions in the weak stratified charge combustion region even under a steady state engine operating condition. This has raised the following problems: Dividing the predetermined quantity of fuel into the two portions decreases the quantity of fuel for each injection and therefore enlarges an offset error (a drift amount in a pulse width--fuel injection quantity characteristics) of the fuel injector valve. This lowers the precision of an air-fuel ratio control and deteriorates exhaust emission characteristics and drivability of the engine as compared with an engine in which only one fuel injection is made in each cycle of the cylinder.
Besides, since the fuel injection quantity on the intake stroke is small, homogeneous air-fuel mixture (charge) becomes excessively lean so that combustion flame is unavoidably extinguished in stratified charge combustion at lean air-fuel ratio, thereby increasing the amount of emission of hydrocarbons (unburned fuel). Particularly, when the two separate fuel injections are made in a low engine load operating range, combustible mixture is formed at the second fuel injection and therefore a large quantity of fuel cannot be injected at the first fuel injection, so that the homogeneous mixture becomes excessively lean.
Furthermore, the two separate fuel injections under the steady state engine operating condition increases the amount of heat generated in a drive unit for driving the fuel injector valve which is of the high voltage-boosting type, or increases production cost for the drive unit if the drive unit includes two systems for suppressing heat generation.